An RFID tag and a method for optimizing a Yagi-Uda antenna for a RFID inlay or integrated circuit is provided. The method produces a tag having three or four non-touching separate, electrically conductive elements located within the same plane, wherein a space of no more than +/−0.10 inches is located from the top to the bottom of the inlay. The method has four main steps and an optional fifth step wherein the steps comprise: 1) first, selecting an electrically conductively first element (which in some cases may be an RFID dipole inlay with an integrated circuit or a conductive element dipole length which is adjusted to a dielectric constant of an applied substrate or form factor); 2) second, generating an electrically conductive second element by taking a mirror image of the electrically conductive first element, trimming off the length and/or width for the applicant's substrate; 3) third, generating a conductive strip (or “third element”) approximately three-quarters the width of the electrically conductive first element and approximately one and a half times the length of the electrically conductive first element and centering the conductive strip along a line of axis of symmetry of the first two elements and on the opposite side of the first element as the electrically conductive second element; 4) fourth, adjusting the distance between the first three elements for optimum performance using Et(x)=EXP[a−x·b]+E∞; and 5) fifth, optionally, if the electrically conductive first element does not contain an IC (integrated circuit) or RFID inlay, then adding an IC or RFID inlay adjacent to/or physically touching the electrically conductive first element along the line of symmetry.
For an RFID tag with a dipole antenna, the orientation of the tag relative to the reader influences the maximum read distance of the tag. In fact, all RFID tags with a dipole antenna have a characteristic relationship between orientation and max read range called a radiation pattern. The shape of a dipole antenna's radiation pattern generally resembles the shape of an hourglass.
One way of making a dipole antenna's radiation pattern directional and increasing gain is by creating a Yagi-Uda antenna. A Yagi-Uda antenna is a directional antenna having a driven element (dipole or folded dipole) and additional parasitic elements (usually called reflector or directors). The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter. This design achieves a substantial increase in the antenna's directionality and gain compared to a simple dipole.
The parasitic elements in a Yagi antenna are mounted parallel to the driven element, with all the elements usually in a line perpendicular to the direction of radiation of the antenna. What effect a parasitic element has on the radiation pattern depends both on its separation from the next element, and on its length.
Attempts have been made to optimize RFID inlay devices. For example, U.S. Pat. No. 7,999,677 to Azuma discloses RFID tags each having an antenna partially raised from a surface of an underlying object by use of embossments in the RFID inlay base structure. Also disclosed are methods of forming the RFID tags and an RFID system utilizing the RFID tags.
Further, U.S. Pat. No. 8,248,240 to Osaki discloses an RFID tag having an inlay which has a base, an antenna formed on the base, and an IC chip. The IC chip is enclosed in a surface mount package and soldered to the antenna and carries out radio communication through the antenna. The RFID tag further includes underfill that fills a gap between the base and the surface mount package, and a sheath protecting material enclosing the entire inlay.
U.S. Pat. No. 8,212,676 to Cullen discloses a radio frequency identification (RFID) tag and method of manufacturing the same. In a preferred embodiment, the RFID tag includes a radio frequency (RF) inlay, the RF inlay including a carrier sheet, an antenna printed on the carrier sheet and a wireless communication device bonded to the antenna. The RFID tag also includes a plastic extrudate, the RF inlay being disposed within the extrudate so that the antenna and the wireless communication device are encapsulated on all sides within the extrudate. Optional metallic reflector and mounting adhesive layers may be laminated onto the underside of the extrudate. The present invention is also directed to an automated method for manufacturing the above RFID tag, such a method involving, in one embodiment, feeding a continuous supply of RF inlays into a cross-head extruder to yield a continuously extruded block and then cutting the block between successive antennae to yield a plurality of individual RFID tags.
However, these patents fail to disclose method for optimizing a Yagi-Uda antenna for an RFID inlay or IC which is efficient and easy to manufacture as is in the present method. Accordingly, a need exists for an improved method for optimizing a Yagi-Uda antenna for an RFID inlay or IC.